Electromagnetic soundings, including CSEM experiments, are conducted by transmitting an electromagnetic signal, typically a low-frequency periodic waveform, into the subsurface, and measuring the electromagnetic response. U.S. Pat. No. 6,603,313 to Srnka and U.S. Patent Publication No. 2003/0050759 (PCT Publication No. WO 03/025803) by Srnka and Carazzone disclose methods for using CSEM measurements to prospect for oil and gas, and to delineate known prospects. To date, marine CSEM surveys have employed horizontal electric dipoles (HED) as the source of electromagnetic fields. U.S. Provisional Patent Application No. 60/500,787 discloses certain advantages to using a vertical electric dipole (VED) transmitting antenna.
An electric dipole source may be made as follows. Two insulated wires are extended from the two output terminals of a power generator capable of supplying electrical power with a desired frequency and waveform. The other end of each insulated wire is connected to an electrode, or the insulation may be stripped from the end and the bare wire becomes the electrode. The current loop is completed in a marine application by the water, the sea bottom, and possibly the air above the water. (One advantage of the VED is that it generates negligible airwave contribution at the receiver, even in shallow water, improving the target signal.) The two electrodes are maintained a fixed distance apart, and the dipole axis is maintained in a horizontal posture in the case of an HED, or a vertical posture in the case of a VED. Marine CSEM experiments require, for efficiency, that the source antenna be towed by a vessel. Current sub-sea CSEM surveys use a horizontally oriented antenna to transmit electromagnetic waves. As illustrated in FIG. 1, one end of the antenna 11 is attached to a tow body 12, which is lowered to the desired water depth via a sub-sea tow cable 13. The subsea tow body 12 is more than just an anchor point for the towline. It provides a place to contain the sub-sea electrical components necessary for generating the electromagnetic source wave, and also may contain communication systems, positioning systems, speed of sound measuring devices, altimeters and the like. A winch (not shown), attached to a surface vessel 14, controls the tow cable. It is currently known how to design the antenna so that it trails horizontally behind the sub-sea tow body. The depth of the forward end of the HED is governed by the depth and location of the sub-sea tow body, and the antenna is kept at a constant depth (relative to the tow body) throughout its length by designing the antenna to be neutrally buoyant. Real time monitoring feeds precise location and depth coordinates back to the boat to ensure proper depth of the antenna. These methods will not work for a VED, because natural fluid dynamics will cause a towed object to orient itself in the position of minimum hydraulic resistance. This will be horizontal in the case of a dipole antenna. The generated electromagnetic fields will be a function of the geometry of the transmitter, i.e., the antenna dipole, and the electrical structures of the earth. By changing the location of the electrodes 15 to a vertical orientation instead of a horizontal orientation, the electromagnetic fields change with it. Although VED's are not so far known to have been towed in this manner, it is obvious that practical means must be found to maintain the dipole in a vertical orientation as it is towed through the water. The present invention satisfies this need